Method and apparatus for adjusting lithium oxide concentration in wafers

ABSTRACT

In one embodiment, lithium oxide concentration in wafers is adjusted by placing the wafers in a vessel. Vapor of a lithium oxide source is provided and absorbed by the wafers, thereby adjusting the lithium oxide concentration in the wafers. In another embodiment, a two-phase lithium-rich source is placed between wafers such that space in the process chamber is efficiently utilized. In another embodiment, the wafers to be processed are placed in a section of a process chamber (e.g., process tube). Lithium oxide is introduced on end of the process chamber. Carrier gas is also introduced on that end of the process chamber to carry the lithium oxide into the section of the process chamber where the wafers are located. By adjusting the partial pressure of lithium oxide in the process chamber, the rate at which lithium oxide is absorbed by the wafers is controlled.

REFERENCE TO RELATED APPLICATIONS

The present application is related to the following commonly-owneddisclosures, which are incorporated herein by reference in theirentirety:

-   -   (a) U.S. application Ser. No. 10/113,377, filed Mar. 29, 2002,        now U.S. Pat. No. 6,652,644, entitled “ADJUSTING LITHIUM OXIDE        CONCENTRATION IN WAFERS USING A TWO-PHASE LITHIUM-RICH SOURCE,”        filed by Gregory D. Miller and Janos Lazar on the same day as        the present application; and    -   (b) U.S. application Ser. No. 10/113,876, filed Mar. 29, 2002,        pending, entitled “CONTROLLED PARTIAL PRESSURE TECHNIQUE FOR        ADJUSTING LITHIUM OXIDE CONCENTRATION IN WAFERS,” filed by        Gregory D. Miller on the same day as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wafer processing, and moreparticularly to methods and apparatus for adjusting the lithium oxideconcentration in wafers.

2. Description of the Background Art

Lithium tantalate (LiTaO₃) and lithium niobate (LiNbO₃) are widely usedmaterials for fabricating nonlinear optical devices because of theirrelatively large electro-optic and nonlinear optical coefficients. Thesenonlinear optical devices include wavelength converters, amplifiers,tunable sources, dispersion compensators, and optical gated mixers, forexample.

Stoichiometric lithium tantalate (SLT) and congruent grown lithiumtantalate (CLT) are two types of lithium tantalate wafers. An example ofa lithium niobate wafer is the so-called congruent grown lithium niobate(CGN). It has been shown that SLT has better lifetime and ferroelectricproperties than CLT and CGN in nonlinear optical devices; e.g., see“Crystal Growth and Low Coercive Field 180° Domain SwitchingCharacteristics Of Stoichiometric LiTaO₃,” Applied Physics Letters, Nov.23, 1998, Vol. 73, Number 21, by K. Kitamura et al. However, althoughSLT has desirable properties, SLT wafers are relatively difficult toobtain. In contrast, CLT wafers are produced in large quantities bycommercial suppliers and are thus widely available.

One way of fabricating SLT wafers is by the vapor transportequilibration method described in U.S. Pat. Nos. 4,071,396 and4,071,323, which are both issued to Robert L. Holman (“Holman”). U.S.Pat. Nos. 4,071,396 and 4,071,323 are incorporated herein by referencein their entirety. In Holman, a target wafer is exposed to lithium oxide(Li₂O) vapor produced by heating a mass of a lithium-rich two-phasepowder. The two-phase powder produces a constant vapor pressure oflithium oxide equal to the vapor pressure of stoichiometric lithiumtantalate. The wafer, which is initially deficient in lithium oxide,absorbs lithium oxide from the vapor until it reaches the stoichiometriccomposition (i.e., lithium oxide concentration of 50 mol %, tantalumpentoxide concentration of 50 mol %). At that point, the vapor pressureof stoichiometric lithium tantalate over the surface of the wafer equalsthe vapor pressure of the surrounding lithium oxide, thereby reaching aprocess equilibrium and stopping the diffusion of lithium oxide into thewafer.

The aforementioned Holman process has several disadvantages. Onedisadvantage is that the volume of the two-phase lithium-rich powder maybe greater than that of the target wafer. This may limit the throughputof commercially available furnaces for performing the process in thatthere will be less available process tube flat zone left available forwafers. Another disadvantage is that the wafers are placed in closeproximity to a large amount of crumbly two-phase lithium-rich powder,increasing the potential for surface contamination. Still anotherdisadvantage of the Holman process is that it requires aspace-inefficient containment vessel to eliminate pressure gradients.Yet another disadvantage of the Holman process is that it restricts theresulting lithium oxide concentration in the wafer to be that ofstoichiometric lithium tantalate. Although there are applications wherea stoichiometric composition is desirable, the Holman process cannot beused in other applications where the lithium oxide concentration in thewafer is preferably below 50 mol %.

SLT wafers can also be fabricated using the double-crucible Czochralski(DCC) growth method. In the DCC growth method, a boule of lithiumtantalate is pulled from a melt in the center crucible of a concentriccrucible pair. The lithium oxide concentration in the melt is chosensuch that the initially grown material is of the stoichiometriccomposition. As the boule grows and is pulled from the melt, astoichiometric mixture of lithium oxide and tantalum pentoxide (Ta₂O₅)powder is poured into the outer crucible at a rate carefully controlledto equal the rate of crystal growth.

Crystal growth rate using the DCC growth method is a fraction of thegrowth rate achievable using congruent growth methods. Thus, SLT wafersfabricated using the DCC growth method are not as cost effective as CLTwafers. Also, in the DCC growth method, striations in the resultingwafers are difficult to suppress, causing variations in opticalproperties from wafer to wafer.

Another way of fabricating SLT wafers is by the Czochralski growth froma lithium-rich melt (LRM) method. In the LRM growth method, only afraction (e.g., approximately 10%) of the melt is used to produce thestoichiometric boule. Because continued growth after using the fractionof the melt results in rapid deviation from the stoichiometriccomposition, the melt is frequently recycled. Tantalum pentoxide powderis added to the recycled melt to achieve the appropriate lithium oxideto tantalum pentoxide concentration for the next growth run.

The LRM growth method grows material at a slower rate than congruentgrowth methods, and has the additional throughput reduction associatedwith the time required to recycle the used melt. Further, becauseaccurate measurement of lithium oxide concentration in a used melt isdifficult, approximations are made to determine the lithium oxideconcentration in the melt. This results in variations in the amount ofuseful grown material from boule to boule.

From the foregoing, an improved technique for adjusting the lithiumoxide concentration in wafers is highly desirable. Ideally, such atechnique should also allow production of SLT wafers in large quantitiesand at a relatively low cost.

SUMMARY

The present invention relates to an improved technique for adjusting thelithium oxide concentration in wafers. In one embodiment, the lithiumoxide concentration is adjusted to stoichiometric concentration. Inother embodiments, the lithium oxide concentration is adjusted to avalue below stoichiometric concentration.

In one embodiment, the present invention is employed to adjust thelithium oxide concentration in CLT wafers to stoichiometricconcentration, thereby converting the CLT wafers to SLT wafers. As canbe appreciated, using generally available CLT wafers to create SLTwafers results in manufacturing cost savings. Further, using theteachings of the present invention, SLT wafers can be fabricated fromCLT wafers in relatively large quantities.

In one embodiment, a lithium oxide source is placed in a vessel alongwith wafers to be processed. The vessel is heated to transform thelithium oxide source into vapor, which is then absorbed by the wafers.In one embodiment, the lithium oxide source includes lithium oxidepowder.

In one embodiment, the vessel is a sealed vessel and components withinthe vessel are resistant to lithium oxide. Thus, when the vessel isheated, all of the created lithium oxide vapor is absorbed by thewafers. The resulting lithium oxide concentration in the wafers can beadjusted by controlling the amount of lithium oxide placed in thevessel. In other embodiments, the vessel is a leaky vessel. In thoseembodiments, excess lithium oxide source is placed in the vessel tocompensate for the leak.

In one embodiment, a two-phase lithium-rich source (e.g., two-phaselithium tantalate powder) source is placed between wafers such thatspace in the process chamber is efficiently utilized.

In one embodiment, the wafers to be processed are placed in a section ofa process chamber (e.g., process tube). Lithium oxide is introduced onone end of the process chamber. Carrier gas is also introduced on thatend of the process chamber to carry the lithium oxide into the sectionof the process chamber where the wafers are located. By adjusting thepartial pressure of lithium oxide in the process chamber, the rate atwhich lithium oxide is absorbed by the wafers is controlled.

These and other features and advantages of the present invention will bereadily apparent to persons of ordinary skill in the art upon readingthe entirety of this disclosure, which includes the accompanyingdrawings and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a process environment in accordancewith an embodiment of the present invention.

FIGS. 2A and 2B show a side cross-sectional view and a front x-ray view,respectively, of a vessel in accordance with an embodiment of thepresent invention.

FIG. 3 shows a flow diagram of a process for adjusting the lithium oxideconcentration in a wafer in accordance with an embodiment of the presentinvention.

FIG. 4 shows a side cross-sectional view of a vessel in accordance withan embodiment of the present invention.

FIGS. 5A and 5B schematically show side cross-sectional views of avessel in accordance with an embodiment of the present invention.

FIGS. 6A-E schematically illustrate various ways of providing lithiumoxide vapor in accordance with embodiments of the present invention.

FIG. 7A shows a schematic diagram of a vessel containing target wafersinterspersed by two-phase lithium-rich sources, in accordance with anembodiment of the present invention.

FIG. 7B shows a flow diagram of a process for adjusting the lithiumoxide concentration in wafers using interspersed lithium oxide sources,in accordance with an embodiment of the present invention.

FIG. 8 shows a process environment in accordance with another embodimentof the present invention.

FIG. 9 shows a flow diagram of a process for adjusting the lithium oxideconcentration in a wafer in accordance with an embodiment of the presentinvention.

FIG. 10 shows a graph illustrating the pressure differential between atarget wafer and a lithium oxide source.

FIGS. 11, 12, and 13 show schematic diagrams of process environments inaccordance with embodiments of the present invention.

The use of the same reference label in different drawings indicates thesame or like components. Unless otherwise noted, the aforementioneddrawings are not drawn to scale.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus and/or methods, to provide a thoroughunderstanding of embodiments of the invention. One skilled in therelevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with otherapparatus, systems, methods, components, materials, parts, and/or thelike. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the invention.

Referring now to FIG. 1, there is shown a schematic diagram of a processenvironment 100 in accordance with an embodiment of the presentinvention. As shown in FIG. 1, a vessel 104 is placed inside processtube 102 of a furnace 101. One or more vessels 104 may be used dependingon the size of furnace 101 and the dimensions of each vessel. Specificimplementations of vessel 104 include vessels 104A, 104B, 104C, 104D,and 604, all of which are described later on below.

Vessel 104 is moved in and out of process tube 102 using a boat 105,which enters the furnace through opening 103. During wafer processing,heating elements (not shown) surrounding process tube 102 are energizedto raise the temperature inside process tube 102 and thereby heat vessel104. As can be appreciated, the just described process environment isexemplary; other techniques for heating vessel 104 may also be usedwithout detracting from the merits of the present invention. Forexample, the use of a process tube is not specifically required becauseother heated process chambers may also be used.

FIGS. 2A and 2B show a side cross-sectional view and a front x-ray view,respectively, of a vessel 104A in accordance with an embodiment of thepresent invention. As mentioned, vessel 104A is a specific embodiment ofvessel 104 shown in FIG. 1. Vessel 104A includes a housing 201, which issealed by a cap 202 on one end and by a cap 203 on the other end. Caps202 and 203 are threaded onto housing 201. In one embodiment, vessel104A has a dimension D1 of 6.750 inches, a dimension D2 of 6.000 inches,and a dimension D3 of 5.250 inches.

Housing 201, cap 202, and cap 203 provide mechanical containment of anampoule 221, which may otherwise expand and potentially rupture due tothe expansion of contained gases at elevated temperatures.

Vessel 104A further includes an ampoule 221 which is sealed by a cap222. In one embodiment, cap 222 is welded onto ampoule 221 prior towafer processing to provide a hermetic seal, thereby preventing vaporfrom escaping out of ampoule 221. Arc welding or TIG welding may beemployed to seal the ampoule. After wafer processing, the welding beadsattaching cap 222 to ampoule 221 are cut to allow the two components tobe separated.

Each wafer to be processed, such as a wafer 112, is placed on a slot ofwafer carrier 231. Wafer carrier 231 holds the wafers inside ampoule 221during wafer processing. In this embodiment, vessel 104A has thecapacity to process fourteen 4-inch diameter wafers at a time. As can beappreciated, vessel 104A may be extended to accommodate additionalwafers; enlarging vessel 104A allows it to accommodate larger wafers. Itis estimated that vessel 104A, and other vessels in the presentdisclosure, may be scaled to accommodate around 300 wafers at a time.This is in marked contrast to other types of vessels that can onlyprocess a few wafers at a time.

Still referring to FIGS. 2A and 2B, a box 210 for holding a lithiumoxide source (e.g., lithium oxide powder) is placed inside ampoule 221.Box 210 is placed underneath the wafers in this embodiment. When vessel104A is heated, lithium oxide vapor from the lithium oxide sourceescapes from the top portion of box 210 and into the surrounding volume131.

Ampoule 221, cap 222, and all components within volume 131 such as box210 and carrier 231 (collectively “ampoule components”) are preferablymade of material that will not absorb lithium oxide. This helps ensurethat all of the lithium oxide source in box 210 is converted to vaporand absorbed by the wafers inside volume 131. Because vessel 104A issealed, the wafers all achieve the same end-point vapor pressure,thereby allowing the wafers to have similar lithium oxide concentration.

In one embodiment, a platinum-iridium alloy is the preferred materialfor making ampoule components. Other materials that may be used forfabricating ampoule components include, without limitation, platinum andiridium. The platinum in the platinum-iridium alloy prevents absorptionof lithium oxide, while the iridium in the alloy retards the growth ofgrain boundaries at high temperatures. As can be appreciated, grainboundaries are potential leakage paths and are thus advantageouslyminimized. To further prevent the development of leakage paths, ampoule221 and cap 222 have sufficient wall thickness, which in one embodimentis about 0.05 inch.

Because platinum reacts violently with silicon carbide, a materialcommonly used in furnaces, the outer surfaces of vessel 104A such ashousing 201, cap 202, and cap 203 are preferably made of a heatresistant material other than platinum. In one embodiment, the outersurfaces of vessel 104A are made of alumina. Of course, other materialsmay also be used (including platinum) depending on the specific furnaceemployed.

FIG. 3 shows a flow diagram of a process 300 for adjusting the lithiumoxide concentration in a wafer in accordance with an embodiment of thepresent invention. Process 300 is described in conjunction with vessel104A; however, process 300 may also be performed using other types ofvessels.

In action 302, wafers (or a single wafer) are placed inside vessel 104A.In one embodiment, the wafers are unpolished CLT wafers such as thosecommercially available from Shin-Etsu Chemical, Inc. of Japan andYamaju, Inc. also of Japan. The initial lithium oxide concentration ineach wafer will vary depending on the manufacturer of the wafer. Forexample, each CLT wafer may have a lithium oxide concentration ofapproximately 48.5 mol % and a tantalum pentoxide concentration ofapproximately 51.5 mol %. Wafers with different lithium oxideconcentration may also be used because, as discussed below, lithiumoxide concentration can be adjusted using the teachings of the presentinvention.

In action 304, a finite amount of lithium oxide source is placed in box210 of vessel 104A (see FIG. 2A). In this embodiment, the lithium oxidesource includes lithium oxide powder. Lithium oxide powder iscommercially available from various companies including VWRInternational. Lithium oxide powder is not as bulky as two-phaselithium-rich powders (e.g., two-phase lithium niobate powder, two-phaselithium tantalate powder), thereby allowing for more wafer space insidethe vessel and improving throughput. Because lithium oxide powderreadily absorbs moisture, action 304 is preferably performed in a dryenvironment.

The amount of lithium oxide powder inside vessel 104A dictates thelithium oxide concentration in the resulting wafers. That is, thelithium oxide concentration in the wafers may be adjusted tostoichiometric concentration or some other concentration by adjustingthe amount of lithium oxide powder in the vessel. A microbalance, forexample, may be used to weigh a precise amount of lithium oxide powder.Because vessel 104A is designed to be leak-free and the ampoulematerials do not absorb lithium oxide, all lithium oxide powder that issubsequently converted to vapor is absorbed by the wafers inside thevessel. Thus, a mass calculation may be performed to determine theamount of lithium oxide powder required to achieve a specific lithiumoxide concentration in the wafers. Experimental methods may also beused. For example, several wafer processing runs using various amountsof lithium oxide powder may be performed; the lithium oxideconcentration of the resulting wafers can then be measured andcorrelated with the amount of lithium oxide powder used.

By adjusting the amount of lithium oxide powder used, the lithium oxideconcentration in wafers may be adjusted to a value between that ofcongruent composition and that of stoichiometric composition. In theevent that more than enough lithium oxide powder is used, the excesslithium oxide powder results in a wafer whose surface consists of amixture of the stoichiometric and lithium-rich phases. However, theinner volume of the wafer remains stoichiometric, and the wafer may bepolished to produce a stoichiometric wafer.

One way of measuring the lithium oxide concentration in a wafer is bymeasuring its Curie temperature. Generally speaking, the Curietemperature of a material is the temperature beyond which the materialloses its ferroelectric properties. Curie temperatures can be measuredusing various known techniques including by differential scanningcalorimeter and electronic tests. The Curie temperature of a wafer isindicative of its lithium oxide-tantalum pentoxide ratio. For example,experimental results indicate that a CLT wafer having a lithium oxideconcentration of 48.5 mol % has a Curie temperature of around 595° C. to605° C. An SLT wafer having a lithium oxide concentration of 50 mol %has a Curie temperature of around 695° C.

In one embodiment, SLT wafers are created by adjusting the lithium oxideconcentration in CLT wafers placed in vessel 104A. This is done by usingan amount of lithium oxide powder that is sufficient to change thelithium oxide concentration in the CLT wafers to 50 mol %. As can beappreciated, using widely available CLT wafers to fabricate SLT wafersresults in manufacturing cost savings. Further, using the teachings ofthe present invention, SLT wafers can be fabricated from CLT wafers inrelatively large quantities.

Other lithium oxide concentrations (e.g., 49.5 mol %) are also possibleby adjusting the amount of lithium oxide powder in vessel 104A.

Continuing with action 306, vessel 104A is closed after the lithiumoxide powder and wafers are placed therein. In this embodiment, vessel104A is hermetically sealed by arc welding cap 222 onto ampoule 221.Note that in embodiments employing other vessels, the vessel may or maynot be hermetically sealed.

In action 308, vessel 104A is heated in a furnace until all of thelithium oxide powder is consumed. As can be appreciated, this allows fora simple and repeatable end-point because the wafers stop absorbinglithium oxide after all of the lithium oxide powder is converted intovapor and absorbed by the wafers. Extending the process time past thepoint where all of the lithium oxide powder is consumed, within reason,does not affect the resulting lithium oxide concentration in the wafers.

In this embodiment, vessel 104A is heated to a temperature high enoughto vaporize lithium oxide powder (e.g., above 660° C.). It is estimatedthat it would take approximately 100 hours to heat a vessel 104A, at anominal furnace temperature of 1350° C., to turn CLT wafers into SLTwafers. The nominal pressure inside vessel 104A during that time isestimated to be around 5 atm to 6 atm. The pressure inside the vessel isprimarily due to expansion of air trapped in the vessel when it washermetically sealed. Of course, specific process parameters such asprocessing time, heating temperature, and the amount of lithium oxidepowder would vary depending on implementation.

In action 310, vessel 104A is opened after wafer processing. In thisembodiment, vessel 104A is opened by cutting the beads of the weld usedto attach cap 222 on ampoule 221. In action 312, the processed wafers(which now have the desired lithium oxide concentration) are removedfrom vessel 104A. The wafers may then be polished in a subsequentpolishing operation.

In one experiment using the just described process 300, CLT wafers whoselithium oxide concentration was originally 48.5 mol % exhibited Curietemperatures above 695° C. and coercive fields in the range of 100-200V/mm after processing. The relatively high Curie temperature andrelatively low coercive fields indicate that the resulting wafers havethe desired near stoichiometric composition. As can be appreciated,process 300 may be fine tuned to achieve optimum results.

In another embodiment of the present invention, a highly reusable vessel104B is employed in process 300 in lieu of vessel 104A. FIG. 4 shows aside cross-sectional view of such a vessel 104B in one embodiment.Vessel 104B includes a housing 401 and a cap 402. Cap 402 is threadedonto housing 401 to create a hermetically sealed volume 403. Unlikevessel 104A, vessel 104B does not include an internal ampoule. Hermeticsealing is achieved in vessel 104B by using a high temperature gasket404 between housing 401 and cap 402. Gasket 404 may be made of alumina,for example. At high temperatures, a gasket 404 of alumina expands toseal the interface between housing 401 and cap 402. As can beappreciated, vessel 104B is a highly reusable vessel in that welding andcutting are not required to create a hermetic seal.

In one embodiment, housing 401 and cap 402 are made of silicon carbide.Silicon carbide is a preferred material for vessel 104B because it canbe used at very high temperatures and does not absorb lithium oxide.Additionally, adjoining silicon carbide components (e.g., housing 401and cap 402) do not permanently bond together even when exposed to veryhigh temperatures.

A wafer carrier such as wafer carrier 231 may be used to hold wafersinside vessel 104B. A box (e.g., box 210) containing a desired amount oflithium oxide powder may be placed under the wafer carrier as in vessel104A.

In another embodiment, a leaky vessel 104C (not shown) is employed inprocess 300 in lieu of vessel 104A. Unlike vessel 104A and vessel 104B,vessel 104C does not maintain a hermetic seal. A vessel 104C may beconstructed using a vessel 104B without gasket material. A vessel 104Cmay also be constructed using a vessel 104A without a hermeticallysealed ampoule (e.g., by not welding cap 222 onto ampoule 221). Othervessels with designed-in or unintentional leaks may also be used, solong as the leak is characterized and compensated for as describedbelow.

When using a vessel 104C in process 300, excess lithium dioxide powderis provided to overcompensate for the leak. The amount of lithiumdioxide powder needed to achieve a desired lithium oxide concentrationdepends on the size of the leak and the total process time. As long asthere is more than enough lithium oxide powder in vessel 104C to achievea stoichiometric composition, the resulting wafer would still have alithium oxide concentration of around 50 mol %. The inventors believethat this is because the excess lithium oxide is trapped near thesurface of the wafer, leaving the bulk of the wafer stoichiometric. Theexcess lithium oxide on the surface of the wafer can be removed insubsequent polishing operations.

When using a leaky vessel such as vessel 104C in process 300, the leakyvessel is heated for an amount of time sufficient to achievestoichiometric composition. Because there is an excess amount of lithiumoxide powder in the leaky vessel to compensate for the leak, process 300may be terminated before the entirety of the lithium oxide powder isconsumed. The amount of time sufficient to achieve stoichiometriccomposition may be calculated or empirically determined.

In another embodiment, a vessel 104D, which is designed to leak at lowtemperatures and to seal at high temperatures, is employed in process300 in lieu of vessel 104A. At temperatures too low to transform lithiumoxide powder into vapor, vessel 104D leaks to allow air to escape out,thereby lowering the pressure inside the vessel. At temperatures highenough to vaporize lithium oxide powder (e.g., at wafer processingtemperature), vessel 104D seals to prevent lithium oxide vapor fromleaking out of the vessel. As can be appreciated, a vessel 104D operatesat lower pressures compared to a vessel 104A. This allows vessel 104D tohave thinner walls, which translates to lower weight, thus making vessel104D easier to handle in a fabrication facility.

FIG. 5A schematically shows the various components of a vessel 104D inaccordance with an embodiment of the present invention. A cap 503threads onto an ampoule 504 to create a volume 506 where wafers andlithium oxide powder are placed during wafer processing. Ampoule 504 andcap 503 slide into a housing 505. A cap 501 threads onto housing 505 tocontain ampoule 504. A spacer 502 is placed between cap 501 and cap 503to seal volume 506 at high temperatures. FIG. 5B schematically showsvessel 104D as configured for wafer processing. Wafers, a wafer carrier,and a container for holding lithium oxide powder placed in volume 506during wafer processing are not shown for clarity.

All components of vessel 104D except spacer 502 are preferably made of aheat resistant material that will not absorb lithium oxide. An exampleof such a material is silicon carbide. Silicon carbide has a relativelylow expansion coefficient and will not substantially expand when heated.Spacer 502, on the other hand, is made of a material that will readilyexpand when exposed to elevated temperatures. In one embodiment, spacer502 is made of alumina.

At the initial stages of wafer processing when vessel 104D is still atrelatively low temperatures, gases in volume 506 will leak through thethreads of vessel 104D. As the temperature begins to rise, spacer 502will expand until it provides a tight seal inside vessel 104D. At thatpoint, volume 506 will be sealed and all of the lithium oxide powdercontained therein will be vaporized and absorbed by the wafers. Thus,similar to vessel 104A, vessel 104D may be used to adjust the lithiumoxide concentration in wafers by placing a finite amount of lithiumoxide powder in volume 506 and thereafter heating the vessel.

Alternative ways of providing lithium oxide vapor are now described withreference to the schematic diagrams of FIGS. 6A-6E. In FIG. 6A, acontainer 611 includes an envelope 612. Container 611 may be an ampouleor a reusable vessel containing one or more wafers 614 to be processed,for example. Envelope 612 contains lithium oxide powder and provides amembrane through which only lithium oxide vapor can diffuse. That is,envelope 612 prevents lithium oxide powder from escaping into the volumeof container 611. Envelope 612 may be made of platinum or other hightemperature material. Referring to FIG. 6B, grain boundaries 613 of aplatinum envelope 612 grow when heated to elevated temperatures. Whengrain boundaries 613 grow to sufficient size, they will allow lithiumoxide vapor to diffuse through them. Lithium oxide vapor will continueto diffuse through grain boundaries 613 and reach wafer 614 as long asthe vapor pressure inside envelope 612 is greater than the vaporpressure outside envelope 612.

In FIG. 6C, a micro-pore screen 615 is provided between a lithium oxidepowder 617 and one or more wafers 614. Micro-pore screen 615 includesmany small holes, denoted as micro-pores 616 in FIG. 6D, to allowlithium oxide vapor to pass through while preventing lithium oxidepowder from escaping to the wafer side of container 611. Similarly, ascreen with a single pinhole may also be used.

In FIG. 6E, a small-diameter tube 619 is used as the only connectingelement between container 611 and a powder container 618. Powdercontainer 618 contains lithium oxide powder 617. The temperature of tube619 is controlled independent of container 611 and powder container 618.That is, container 611 and powder container 618 are heated separatelyfrom tube 619. For example, container 611 and powder container 618 maybe heated inside a process chamber, while tube 619 is located outsidethe process chamber. Cooling and heating mechanisms may be coupled totube 619. The manner in which the temperatures of container 611, tube619, and powder container 618 are controlled does not affect theefficacy of the set-up of FIG. 6E.

Before container 611 and powder container 618 reach thermal equilibrium,it is possible for lithium oxide powder to flow into container 611 alongwith lithium oxide vapor. The idea behind the set-up of FIG. 6E is toprevent lithium oxide vapor from traveling into container 611 untilcontainer 611 and powder container 618 reach thermal equilibrium. In oneembodiment, this is done by keeping tube 619 at a temperature lower thanthat of powder container 618 until thermal equilibrium is reached. Doingso allows lithium oxide vapor to condense in tube 619 and prevent thepassage of lithium oxide powder into container 611. When thermalequilibrium is reached, tube 619 is allowed to be heated to thermalequilibrium (e.g., by turning OFF its cooling mechanism and then heatingthe tube), thereby enabling lithium oxide vapor to flow through tube 619and enter container 611.

In another embodiment of the present invention, the wafers to beprocessed (“target wafers”) are placed in a vessel interspersed bytwo-phase lithium-rich sources. To maximize wafer space in the vessel,each two-phase lithium-rich source preferably has compact dimensions anda large surface area. The vessel is preferably sealed but may also beleaky. This embodiment is now further described with reference to FIG.7A.

FIG. 7A shows a schematic diagram of a vessel 604 containing targetwafers 602 interspersed by two-phase lithium-rich sources 601. Vessel604 may be a sealed or leaky vessel. A wafer carrier 231 holds targetwafers 602 inside vessel 604.

In FIG. 7A, each two-phase lithium-rich source 601 may be a sinteredtwo-phase lithium-rich lithium tantalate wafer, which may be a mixtureof stoichiometric lithium tantalate (LiTaO₃) and a lithium-rich phase(Li₃TaO₄) The just mentioned sintered wafers need only be large enoughto contain enough excess lithium oxide for one run, after which they arecrushed and recycled with the addition of lithium carbonate powder tocreate new sintered wafers. If vessel 604 is sealed, the sintered wafersmay have dimensions similar to that of the target wafers. This allowsfor nearly 50% utilization of a process tube flat zone volume, resultingin significant throughput improvement compared to other techniques. Ifvessel 604 is a leaky vessel, the volume of the sintered wafers may beincreased to compensate for the leak.

A thick disk of sintered two-phase lithium-rich lithium tantalate powdermay also be used as a two-phase lithium-rich source 601.

FIG. 7B shows a flow diagram of a process 700 for adjusting the lithiumoxide concentration in a wafer using interspersed two-phase lithium-richsources. In action 702, target wafers 602 are placed inside a vessel604. In one embodiment, target wafers 602 are unpolished CLT wafers.Other wafers having lithium oxide concentration below 50 mol % may alsobe used.

In action 704, two-phase lithium-rich sources 601 are placed betweentarget wafers. In action 706, vessel 604 is closed.

In action 708, vessel 604 is placed and heated in a furnace. Vessel 604is heated to a temperature high enough to emanate lithium oxide vaporfrom the two-phase lithium-rich sources 601 inside vessel 604. Targetwafers 602 will absorb lithium oxide from the two-phase lithium-richsources 601 until the vapor pressure of the surrounding lithium oxideequals the vapor pressure of stoichiometric lithium tantalate over thesurface of the target wafers. At that time, vapor transport will reachequilibrium and target wafers 602 will stop absorbing lithium oxide.Also at that time, target wafers 602, which were originally CLT wafershaving a lithium oxide concentration of approximately 48.5 mol %, willhave a final lithium oxide concentration of about 50 mol %(stoichiometric concentration).

In action 710, vessel 604 is opened. In action 712, target wafers 602are removed from vessel 604. Target wafers 602 may then be polished in asubsequent polishing operation.

FIG. 8 shows a process environment 800 in accordance with anotherembodiment of the present invention. In process environment 800, aprocess tube 802 of a furnace is coupled to a chamber 804 via a massflow controller 851 and plumbing 852. Chamber 804 includes a container810 containing a source of lithium oxide. In one embodiment, container810 contains lithium oxide powder. Chamber 804, and the other chambersdisclosed herein, may be any type of sealed cavity that can be heated totemperatures high enough to vaporize lithium oxide powder. For examplechamber 804 may be a tank, a vessel, a sealed section of a process tube,etc. When chamber 804 is heated (e.g., to approximately 700° C.),lithium oxide powder in container 810 is transformed into vapor. Thelithium oxide vapor flows through mass flow controller 851 and intoprocess tube 802. Mass flow controller 851 and plumbing 852 are heatedto prevent the lithium oxide vapor from precipitating.

Process tube 802 is also coupled to a tank 871 via a mass flowcontroller 881 and plumbing 882. Tank 871 contains a heated carrier gas,which in this embodiment includes oxygen. The carrier gas flows throughmass flow controller 881 and into process tube 802. Oxygen is apreferred carrier gas because lithium tantalate wafers are, generallyspeaking, metal oxide compounds; using an oxygen carrier gas replenishesthe oxygen lost by the wafers during processing. Other carrier gases mayalso be used. For example, clean dry air (CDA) may be used as a carriergas depending on implementation.

Inside process tube 802, wafers 812 (the wafers to be processed) sit ona wafer tray 831. Lithium oxide vapor entering process tube 802 arecarried over wafers 812 by the carrier gas from tank 871. As will beexplained below, the carrier gas also allows for adjustment of thepartial pressure of lithium oxide in process tube 802. A processor 821is coupled to mass flow controllers 851 and 881 to control the amount oflithium oxide vapor and carrier gas entering process tube 802. Processor821 may be a computer, a programmable logic controller, microcontroller,etc. Of course, mass flow controllers 851 and 881 may also be manuallycontrolled depending on implementation.

FIG. 9 shows a flow diagram of a process 900 for adjusting the lithiumoxide concentration in a wafer using a process environment 800. As willbe explained later on below, process 900 may also be used with otherprocess environments.

In action 902, the wafers to be processed are placed in process tube802. In one embodiment, the wafers are CLT wafers. Other wafers havinglithium oxide concentration below 50 mol % may also be used.

In action 904, the heating elements (not shown) of process tube 802 areenergized to raise the temperature in the process tube (e.g., to 1350°C.).

In action 906, lithium oxide vapor is flown from chamber 804 to processtube 802. The amount of lithium oxide vapor flowing into process tube802 is metered by mass flow controller 851.

In action 908, carrier gas is flown from tank 871 to process tube 802.The amount of carrier gas flowing into process tube 802 is metered bymass flow controller 881.

In action 910, the partial pressure of lithium oxide in process tube 802is controlled by controlling the mass flow rates of lithium oxide vaporand/or carrier gas. As is well known, the ratio of mass flow rates of afirst gas and a second gas sets the mass ratio in the mixture of the twogases. Thus, by reading mass flow controllers 851 and 881, the partialpressure of lithium oxide in the process tube can be calculated. Such apartial pressure calculation may be performed by processor 821, forexample. Adjusting the mass flow rate of the carrier gas and/or lithiumoxide vapor, by throttling their respective mass flow controllers,allows for control of the partial pressure of lithium oxide in processtube 802.

In one embodiment, the partial pressure of lithium oxide in process tube802 is increased over a period of time to minimize the pressuredifferential between wafers 812 and the surrounding atmosphere, therebypreventing strain-related damage to the surface of the wafers. At thebeginning of the process, the partial pressure of lithium oxide inprocess tube 802 is set at a low value close to the beginning lithiumtantalate vapor pressure over the surface of wafers 812. As wafers 812absorb lithium oxide, the lithium tantalate vapor pressure over theirsurface begins to increase. The partial pressure of lithium oxide inprocess tube 802 is then correspondingly increased by adjusting massflow controller 851 and/or mass flow controller 881. The optimum rate ofincrease of lithium oxide partial pressure may be calculated orempirically determined.

FIG. 10 shows a graph further illustrating the above concept. As shownin FIG. 10, the vapor pressure over the surface of a target wafer startsat a minimum value dictated by its initial lithium oxide concentration,which is approximately 48.5 mol % in this example. As the wafer absorbslithium oxide, the partial pressure of lithium oxide inside process tube802 is increased to track the increasing vapor pressure over the surfaceof the wafer. This results in a small pressure differential between thewafer and the surrounding lithium oxide. The inventors believe that ahigh pressure differential between the wafer and the surrounding lithiumoxide results in very high lithium oxide absorption rates. This cancause strain-related damage at the surface of the wafer in someapplications. Thus, by minimizing the pressure differential, the presentembodiment improves the quality of the resulting wafers.

Continuing with action 912, process 900 is terminated when wafers 812reach a target composition, which may be non-stoichiometric (e.g., 49.5mol %). Once the target composition is reached, parameters of process900 may be ramped down to minimize defects. The process time of process900 may be calculated or empirically determined. Other processparameters such as mass flow controller settings may also be empiricallydetermined to suit specific applications.

FIG. 11 shows a schematic diagram of a process environment 1100 inaccordance with another embodiment of the present invention. In processenvironment 1100, wafers are processed as in process 900 except that thelithium oxide vapor is created by heating a solution in chamber 1104.Process environment 1100 has the added benefit of not having to heatplumbing external to process tube 802 because, as explained below, thesolution is vaporized in the process tube.

In one embodiment, the solution in chamber 1104 includes lithiumhydroxide and water. The solution is pumped at room temperature throughplumbing 1102, mass flow controller 1101, and an extension tube 1103.Extension tube 1103 protrudes into a region of process tube 802 wherethe temperature is sufficient to vaporize the solution. The length ofextension tube 1103 depends on the temperature in the region of processtube 802 where extension tube 1103 protrudes. Additional heatingelements may be placed in that process tube region to minimize thelength of extension tube 1103.

The carrier gas from tank 871 (e.g., heated oxygen) carries theresulting steam and lithium hydroxide vapor into the hot zone of processtube 802. There, the temperature is sufficient to dissociate the lithiumhydroxide to form lithium oxide as described by the following reaction,${2{LiOH}}\overset{heat}{\rightarrow}{{{Li}_{2}\quad O} + {H_{2}O}}$By controlling the concentration of lithium hydroxide in the solutionand the mass flow rates of the solution and/or the carrier gas, thepartial pressure of lithium oxide in the hot zone where wafers 812 arelocated can be controlled. The resulting lithium oxide vapor is absorbedby wafers 812 as in process 900.

FIG. 12 shows a schematic diagram of a process environment 1200 inaccordance with another embodiment of the present invention. In processenvironment 1200, wafers are processed as in process 900 except that thelithium oxide vapor is created from the reaction of an oxygen carriergas with a metallo-organic gas that contains lithium. A metallo-organicgas, generally speaking, is an organic molecule with a metallic ionattached to it. Such metallo-organic gases are commonly used in metalorganic chemical vapor deposition (MOCVD) processes.

Referring to FIG. 12, tank 1271 supplies a lithium-containingmetallo-organic gas to process tube 802 via plumbing 1202 and mass flowcontroller 1201. The carrier gas from tank 871, which is oxygen in thisembodiment, carries the metallo-organic gas into the hot zone of processtube 802. There, lithium from the metallo-organic gas reacts with oxygento form lithium oxide. Other by-products of the reaction include carbondioxide and carbon monoxide from the reaction of oxygen with carbon. Bycontrolling the mass flow rates of the metallo-organic gas and oxygen,the partial pressure of lithium oxide in the hot zone where wafers 812are located can be controlled. The resulting lithium oxide vapor isabsorbed by wafers 812 as in process 900.

Process environment 1200, like process environment 1100, has the addedbenefit of not having to heat plumbing external to process tube 802because the lithium oxide vapor is created in process tube 802.Additionally, process environment 1200 uses a gas as a lithium source.As can be appreciated, gases are easier to control in a processenvironment compared to liquids.

FIG. 13 shows a process environment 1300 in accordance with anotherembodiment of the present invention. In process environment 1300, wafersare processed as in process 900 except that the lithium oxide vapor iscreated from a lithium oxide source located inside a process tube 1302.

Referring to FIG. 13, a lithium oxide source is provided inside a vessel1304. Vessel 1304 includes a pinhole 1306 of known diameter. Vessel 1304is placed on a zone of process tube 1302 where the temperature iscontrolled by heating elements 1321. Wafers 812 are on another zone ofprocess tube 1302 where the temperature is controlled by heatingelements 1341. As can be appreciated, heating elements 1321 and 1341 aredepicted in FIG. 13 for illustration purposes only; the type of heatingarrangement does not affect the efficacy of the present invention.

In one embodiment, the lithium oxide source in vessel 1304 includeslithium oxide powder. Lithium oxide at a certain temperature has acorresponding pressure. Thus, when vessel 1304 is heated (e.g., above660° C.), the resulting lithium oxide vapor will come out of pinhole1306 at a certain pressure. The temperature of vessel 1304 and the sizeof pinhole 1306 dictate the mass flow rate of lithium oxide vapor out ofvessel 1304 and into process tube 1302, independent of the mass orsurface area of lithium oxide powder inside the vessel. The temperatureof vessel 1304 can be controlled by controlling heating elements 1321 orby moving vessel 1304 to different sections of process tube 1302.

In process environment 1300, a carrier gas carries the escaping lithiumoxide vapor from vessel 1304 to the hot zone of process tube 1302 wherewafers 812 are located. The carrier gas (e.g., oxygen) comes from tank871 and flows through mass flow controller 881. By controlling the massflow rate of the carrier gas and the temperature of vessel 1304, thepartial pressure of lithium oxide vapor surrounding wafers 812 can becontrolled. Lithium oxide vapor is absorbed by wafers 812 as in process900.

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure. Thus,the present invention is limited only by the following claims.

1. A method for adjusting lithium oxide concentration in a wafer, themethod comprising: creating lithium oxide vapor from a lithium oxidesource in a sealed cavity; and absorbing the lithium oxide vapor into awafer to obtain a final lithium oxide concentration.
 2. The method ofclaim 1 wherein the lithium oxide source includes lithium oxide powder.3. The method of claim 1 wherein the final lithium oxide concentrationis about 50 mol %.
 4. The method of claim 1 wherein the final lithiumoxide concentration is less than 50 mol %.
 5. The method of claim 1wherein the entirety of the lithium oxide source is consumed duringprocessing of the wafer.
 6. The method of claim 5 wherein the lithiumoxide source is of an amount that will result in the final lithium oxideconcentration in the wafer to be approximately 50 mol %.
 7. The methodof claim 5 wherein the lithium oxide source is of an amount that willresult in the final lithium oxide concentration in the wafer to be lessthan approximately 50 mol %.
 8. The method of claim 1 wherein the sealedcavity includes a hermetically sealed vessel.
 9. The method of claim 1wherein the lithium oxide source consists essentially of lithium oxide.10. A method of adjusting lithium oxide concentration in a wafer, themethod comprising: creating lithium oxide vapor from a lithium oxidesource in a sealed cavity, the lithium oxide source being of apredetermined amount to compensate for a leak in the cavity; andabsorbing the lithium oxide vapor into a wafer to obtain a final lithiumoxide concentration.
 11. The method of claim 10 wherein the lithiumoxide source includes lithium oxide powder.
 12. The method of claim 10wherein the lithium oxide source consists essentially of lithium oxide.